Visual display and hard copy printing

5.3.2. Visual display and hard copy printing

5.3.2.1. Aim The aim of this test is to ensure that: (a) The visual display demonstrates all of the information in the digital image

array. (b) The printing device records an image similar in appearance to the visual display.

5.3.2.2. Frequency The visual display and hard copy printing test must be performed by a

qualified medical physicist at the time of acceptance testing, as part of the end- of-warranty testing, annually and whenever the CT system is serviced in a way that might affect either the visual display or the printing device.

5.3.2.3. Materials

A calibrated photometer with detectors designed to measure luminance (in units of candellas or nit·m –2 ·sr –1 ) and illuminance (in units of lux or lm/m 2 ). The luminance detector should be a small angle acceptance detector or, preferably, a detector with a fibre-optic probe, which allows for luminance measurements directly from the display surface.

Note that measuring luminance and illuminance requires two different detector configurations. Information on measuring these two quantities is available in an ACR manual [30], an AAPM report [49] and in Ref. [50].

5.3.2.4. Test patterns There are two levels of test pattern available: (1) Single, all inclusive, patterns such as the test pattern of the Society of

Motion Pictures and Television Engineers (SMPTE) (Fig. 28) [52–55] and the AAPM Task Group 18 (Fig. 29) [50] Comprehensive QC Test Pattern;

(2) Specialized test patterns for detailed evaluation of displays [49].

FIG. 28. The SMPTE test pattern for displays and hard copy imaging systems.

FIG. 29. The Comprehensive QC Test Pattern of AAPM’s Task Group 18.

The SMPTE test pattern (Fig. 28) is recognized by most medical imaging companies worldwide and has been in use since the mid-1980s. It is described in Refs [51–54] and is suitable for both acceptance testing and QC. The test pattern is usually available on medical imaging systems or from the manufac- turer’s engineering department.

A task group of AAPM (Task Group 18) has published a report on evaluating medical imaging displays [49] and has developed several test patterns including the Task Group 18 Comprehensive QC Test Pattern (Fig. 29). Information regarding the use of this test pattern, as well as an extensive set of test patterns for evaluation of displays, is available on-line [49].

5.3.2.5. Procedures (a) Optimization of display brightness and contrast Optimization of brightness and contrast are carried out using the two

smaller square patches that are inset into the 0 and 100% patches in the grey scale (Fig. 28 [55]). The inset patches are at 5 and 95% of the grey scale, so that they are 5% above pure black and 5% below pure white. If these two patches are visible, then one can be assured that all the information in the digital image is being displayed appropriately. The procedure is as follows:

(1) Display the SMPTE test pattern. (2) Set the window width to encompass the range of values in the SMPTE

test pattern image. This can be determined by displaying a histogram of the pixel values.

(3) Set the window level at either the centre of this range or the lower end of this range, depending on the design of the system. Note that this window width and window level will be referred to in this book as the ‘standard window width and level’, and should be used for all testing, including QC testing.

(4) Turn both the display brightness and the contrast controls to their minimum settings, i.e. completely counterclockwise. (5) Adjust only the brightness control until the area outside of the image and

the 0% patch are just slightly grey, as opposed to absolutely black. (6) Adjust only the contrast control so that the 95% patch can be clearly seen. (7) Adjust only the brightness control so that the 5% patch can be clearly seen.

(8) Confirm that the alphanumeric data in the display are clear and sharp. If they are not, readjust the contrast control until they are sharp. Note that the display is now optimized. The brightness and contrast controls should not be adjusted in the future by the clinical staff. If the clinical images appear to be unsatisfactory, it is now necessary to adjust the window and level settings, but not the brightness and contrast.

(b) Measurement of display luminance and room illuminance levels Room illuminance is first measured using an illuminance meter. With the

display turned off, the meter should be placed on the surface of the display with the detector facing away from the display. Note that the physicist making the measurement is in the FOV of the detector, which may affect the measurement — for example, if a white laboratory coat is worn or if the physicist is standing between the detector and a light source such as a window.

A second measurement of room illuminance should be made (with the display turned off) with the detector at the position of the physician looking at the images and pointed towards the display.

If either of these measurements exceeds the suggested tolerances, the room illuminance conditions must be corrected before proceeding. Note that the room illuminance levels have an impact on the quality and contrast of the image on the display. Consequently, it may be necessary to readjust the brightness and contrast controls of the monitor after measuring the room illuminance levels if these levels are higher than the suggested tolerances.

The display luminance should be measured at three different levels using the SMPTE test pattern and a fibre-optic probe with a photometer. Measure- ments should be made with the fibre-optic probe placed on the display surface and centred in the 0, 50 and 100% patches of the test pattern.

(c) Optimizing hard copy image quality Once the display as been optimized as described above, it is possible to

set up and optimize the hard copy imaging system. Ensure that the window width and level are set to the standard levels, i.e. the same values as those selected for optimizing the display, and proceed as follows:

(1) Display the SMPTE test pattern at the standard window width and level. (2) Expose a film image of the SMPTE test pattern. (3) Using a densitometer, measure the densities of the 0, 10, 40 and 90%

patches of the film image.

(4) The densities should fall within the ranges given in Table 5. If they do not, it will be necessary to adjust the hard copy imaging device to obtain densities within the appropriate ranges. It may be necessary to obtain the assistance of a qualified service engineer to make internal adjustments to the imaging device or to adjust the lookup table.

(d) Colour display settings All of the above procedures focus on the monochromic (black and white)

display characteristics. These are the most critical characteristics in ensuring good image quality for PET/CT images. The colour portion of the display can

be evaluated and adjusted using the SMPTE colour bar pattern, which is widely available. The image of this pattern on the display is compared with a standard image, usually in hard copy form.

5.3.2.6. Analysis Measurements should be compared with suggested tolerances.

5.3.2.7. Suggested tolerances Room illuminance levels must be in the range of 10–50 lx and ideally in

the range of 2–10 lx. Once the room illuminance levels have been optimized, the illuminance measurements made on the SMPTE test pattern should be as shown in Table 6. The ranges of set-up optical densities for the film images of the SMPTE test pattern are given in Table 5.

TABLE 5. DENSITY RANGES FOR FILM IMAGES OF THE SMPTE TEST PATTERN

SMPTE area 

Density range

(cd/m 2 )

5.3.2.8. Corrective action Adjustment of display brightness and contrast can usually be accom-

plished without the assistance of a service engineer. If these adjustments are not clearly visible on the front or back of the display, the service engineer should be contacted for assistance.

The room illuminance levels are critical to the image quality of displays. If the illuminance levels do not meet the suggested tolerances, the problem should be rapidly corrected. This may require changing the location of room lights, putting light-tight shades on windows and shielding the display from light arising from viewing boxes in other parts of the reading room. It is not unusual to have to make major changes in the lighting in a room to adjust the room illuminance to appropriate levels.

The film densities should fall within the ranges given in Table 6. If they do not, it will be necessary to adjust the hard copy imaging device to obtain densities within the appropriate ranges. It may be necessary to obtain the assistance of a qualified service engineer to make internal adjustments to the imaging device or to adjust the lookup table.

Adjustments for colour balance can be made using the appropriate adjustments, usually located near the brightness and contrast controls. On installation, a service engineer may be helpful in assisting and explaining the approach and terminology used by the manufacturer.

TABLE 6. SUGGESTED TOLERANCE LEVELS FOR DISPLAY LUMINANCE

SMPTE area

Luminance

(cd/m 2 )